In the reproductively intact female, the menstrual cycle represents a continuous state of change in terms of the sex steroid environment. This variation in hormone levels can potentially affect nutrient metabolism. The present project aims to determine the effects of the menstrual cycle on nutrient utilization at rest and during exercise (90 mins exercise at 50% vo2 max). This is achieved by combining indirect calorimetry with stable isotope techniques to determine total fat and carbohydrate oxidation and the utilization of both intramuscular fuels (glycogen and triglyceride) and circulating fuels (free-fatty acids and glucose). Simultaneously the circulating hormone environment is determined to ascertain any relationship with patterns of nutrient utilization. Within subject measurements are made during 3 phases of the menstrual cycle; early follicular, mid-late follicular and mid-luteal. These three phases represent the majority of the female menstrual cycle and are characterized by different sex steroid hormone profiles. It is hypothesized that a. Resting free-fatty acid utilization will be greater in the luteal phase vs. Both follicular phases. b. Exercise glucose utilization will be greatest in the early follicular phase and c. Intramuscular fuels will be utilized the most during exercise in the mid-late follicular and luteal phases. The results from this study have implications regarding the effect of female sex steroids in the control of body fuel stores and substrate/hormone interactions.